The photonic and optoelectronic communities have long been interested in the development of tunable delay systems for optical pulses. Such systems are important in both experiments and devices. Optical delay lines are an essential part of most time-resolved optical experiments, including time-domain terahertz technology, ultrafast optics research, time resolved detection, interferometric spectroscopy, optical coherence tomography, most optical pump/probe experiments, and other applications. The development of an optical delay line with long delay range (>1 ns) and high repetition rate (>100 Hz), however, poses a significant challenge. Thus, real world applications, such as long distance time-of-flight sensing and tomographic imaging, have not been feasible.
A simple form of optical delay line consists of a linear actuator, which moves a retro-reflector forward and backward as disclosed by R. F. Fork and F. A. Beissoer in their article “Real-time intensity autocorrelation interferometer,” Appl. Opt. 17, 3534-3535 (1978). A retro-reflector is an instrument used to cause reflected radiation to return along paths parallel to those of their corresponding incident rays. The scanning velocity of such an optical delay line is limited, however, by mechanical inertia of the retro-reflector and the translation stage. The typical scanning speed of these optical delay lines is tens of centimeters per second and the repetition rate is generally limited to tens of hertz. Mechanical inertia also affects the linearity of the delay. At both ends of the delay sweep of the delay line, the motion of the retro-reflector must be slowed and then reversed, preventing the optical delay line from having a true linear delay scan function through the entire range. Additionally, the motion of the linear translation stage may not be sufficiently smooth to provide the desired linearity in the delay scan, or may have undesired hysteresis, particularly at high scan rates.
Various techniques have been developed for high-speed scanning. Although the methods disclosed in a number articles offer high-speed scanning, the delay ranges of their high speed devices are limited. Such techniques include, for example, using a piezoelectric transducer to replace the linear motor, delay lines based on a grating lens, rotation glass blocks, mirror arrays, spiral reflectors piezoelectric fiber stretchers, grating-lens based delay lines, helicoids mirrors, and multi-pass cavities among others.
Using a piezoelectric transducer to replace the linear motor, the delay line may have a kilohertz repetition rate; however, the scanning range is of such a delay line is very limited. A delay line based on a grating-lens was disclosed was disclosed by G. J. Tearney, B. E. Bouma, S. A. Boppart, B. Golubovic, E. A. Swanson, and J. G. Fujimoto in their article “Rapid acquisition of in vivo biological images by use of optical coherence tomography,” Opt. Lett. 17 1408-1410 (1996). This optical delay line was able to increase the scanning speed up to tens of kilohertz. Unfortunately, all of these delay line scanners suffer from both low duty cycles and nonlinearity in optical path-length change.
Piezoelectric fiber stretchers having a repetition rate of 1.2 KHz were demonstrated by K. F. Kwong, D. Yankelevich, K. G. Chu, J. P. Heritage, and A. Dienes in their article “400-Hz mechanical scanning optical delay line,” Opt. Lett. 18, 558-560 (1993), but the scanning range of a delay line of this type is limited and suffers from the birefringence effect. Chi-Luen Wang, Sheng-An Wang, S. C. Wang, and Ci-Ling Pan demonstrated a helicoid mirror based delay line in their article “Rapid and programmable wavelength tuning of an external-cavity diode laser,” Conference on Lasers and Electro-optics (CLEO '98), Vol. 11, paper CWN5 (May 3-8, 1998, San Francisco, Calif.). These delay lines, developed from pulse shaper technology, may achieve a 2 KHz repetition rate at a 3 mm scanning range. They exhibit a severe bandwidth limitation, however, and are very costly to produce. More recently, several other delay line scanning systems have been demonstrated having high duty-cycles at rates above 2 KHz, including: a rotation prism array by N. G. Chen and Q. Zhu in their article “Rotary mirror array for high-speed optical coherence tomography,” Opt. Lett. 27, 607-609 (2002); a rotation mirror array by X. Liu, M. J. Cobb, and X. Li in their article “Rapid scanning all-reflective optical delay line for real-time optical coherence tomography,” Opt. Lett. 29, 80-82 (2004); and a multi-pass cavity by P. L. Hsiung, X. Li, C. Chudoba, I. Hartl, T. H. Ko, and J. G. Fujimoto in their article “High-speed path-length scanning with a multiple-pass cavity delay line,” Appl. Opt. 42, 640-648 (2003). None of the above technologies, however, can provide both tens of centimeter scanning range and a repetition rate in the hundreds of hertz range.
True time delay devices based on switched fiber delay lines or optical coherent transient regenerators able to provide a long delay range (up to micro second delay with bit rates up to GHz) have been demonstrated. Switched fiber delay lines are discussed by A. Goutzoulis, K. Davies, J. Zomp, P. Hrycak, and A. Johnson in their article “Development and field demonstration of a hardware-compressive fiber-optic true-time-delay steering system for phased-array antennas,” Appl. Opt. 33, 8173-8185 (1994) and optical coherent transient regenerators are discussed by K. D. Merkel, and W. R. Babbitt in “Optical coherent-transient true-time-delay regenerator,” Opt. Lett. 15, 1102-1104 (1996). The temporal resolution of these devices was relatively low, however, and the devices suffered significant optical loss.
To overcome the shortcomings of existing optical delay lines, a design of a simple, high-speed, high duty-cycle, long range, and linear optical delay line based on scanning the optical beam along an involute curved reflector is provided. One aspect of the present invention provides a compact optical delay line based on a circular involute optical delay stage. Another aspect includes a reflector with a circular involute profile in the optical delay stage. A further aspect provides a compact delay stage for compact and portable optical time-resolved systems, which may require relatively long time delay at high scanning speeds. Still another aspect of the present invention provides a compact, simple, easily aligned system with features such as high scanning speed, linearity, and zero backlash.